1. Field of the Invention
The present invention relates to a laminate. More particularly, the invention relates to a laminate having a layer made of a polyethylene composition and having an excellent balance among tearability, punchability, heat-sealing strength low-temperature heat sealability, and hot-tack properties.
2. Description of Related Art
There is a trend toward volume reduction in packages/containers for beverages and foods, seasonings, medicines, etc because of the recent problem concerning final refuse disposal, the Recycling Law, and the like. On the other hand, investigations are being made on use of containers made of paper as containers which are easy to burn and have a low calorific value of combustion in incineration. From the standpoint of convenience for use, these packages/containers are desired to be easy to open.
Laminates which have been used as the materials of conventional packages/containers are ones obtained by laminating a polyethylene resin comprising high-pressure-process low-density polyethylene (LDPE), an ethylene/vinyl acetate copolymer (EVA), or the like as a heat-sealable layer resin to a base such as paper or a biaxially oriented polyamide, polyester, polypropylene, or the like, from the standpoint of imparting properties required of containers having heat sealability moisture-proof properties etc.
Recently, however, use of linear low-density polyethylene (LLDPE), in particular, LLDPE produced by polymerization with a metallocene catalyst, has come to be proposed in order to improve the heat-sealing strength, low-temperature heat sealability, hot-tack properties, impact resistance, pinhole-free properties, etc. of those laminates.
The LLDPE produced by polymerization with a metallocene catalyst is characterized by being heat-sealable at low temperatures and having high sealing strength and high hot-tack strength, and is in extensive use as a sealant in light packages, paper containers for liquids, etc. However, in applications involving a punching step, such as paper cups and paper containers, the polyethylene has problems, for example, that it shows poor punchability and cannot be punched and the resin layer stretches disadvantageously, resulting in a poor appearance. When used in binding paper or easy-to-tear packages, the polyethylene has problems, for example, that since it has poor tearability, opening necessitates power and the resin layer stretches.
Many attempts have been made to overcome those problems. For example, a laminate of LLDPE produced by polymerization with a metallocene catalyst and LLDPE produced by polymerization with a Ziegler catalyst has been proposed (see, for example, patent document 1) This laminate has slightly improved tearability due to the LLDPE produced by polymerization with a Ziegler catalyst. However, the elongation attributable to the LLDPE produced by polymerization with a metallocene catalyst still remains uneliminated and, in particular, heat sealability is considerably sacrificed. Consequently, that technique is not a desirable one. On the other hand, a technique has been proposed in which a cycloolefin polymer which is an amorphous resin is added to LLDPE produced by polymerization with a metallocene catalyst (see, for example, patent document 2). However, this technique has a problem concerning a decrease in transparency which occurs upon heating. Furthermore, an invention has been disclosed in which LDPE having a specific swelling ratio is incorporated into LLDPE produced by polymerization with a metallocene catalyst (see, for example, patent document 3). However, this technique has been insufficient in balance between heat-sealing strength and tearability.    Patent Document 1: JP-A-10-24539    Patent Document 2: JP-A-2000-129005    Patent Document 3: JP-A-2000-212339